From First Principles
by Lys ap Adin
Summary: There are rules for situations like this, but maybe they're not the right rules. Shiro x Keith x Lance, get-together fic, nothing heavier than some kissing and Shiro's enormous, overdeveloped sense of responsibility. Though that's pretty danged heavy.


**Title:** From First Principles  
 **Characters/Pairings:** Shiro/Keith/Lance  
 **Summary:** There are rules for situations like this, but maybe they're not the right rules.  
 **Notes:** Get-together fic, nothing heavier than some kissing and Shiro's enormous, overdeveloped sense of responsibility. 5055 words.

* * *

 **From First Principles**

"You don't want to go in there," Pidge says on the way out of the training deck, looking two parts exasperated to one part amused.

Hunk is following and rolls his eyes. "They're at it again," he adds, though he doesn't need to—Shiro can hear the yelling himself, even through the closed doors. "Might as well let them get it out of their systems."

"We're going to hit the lab," Pidge says. "You're welcome to come with us."

The offer is more tempting than Shiro likes to admit, even to himself. If he heads into the training deck, he's volunteering himself to referee yet another round of Keith and Lance's eternal argument. It's the kind of thing that could wear on the patience of a saint, which he is not. The lab sounds like a delightfully peaceful alternative, and there's no saying what sort of technological marvel Pidge and Hunk have up their sleeves now.

Then he hears what sounds a lot like gunfire from the training deck and dismisses the thought, tempting though it is. "You go on without me," he says, rueful. "I'd better go tell them to knock it off."

Pidge nods as if this is only to be expected and Hunk gives him a commiserating smile. "Have fun."

"Thanks," Shiro tells him, palming the door open as they head off.

He finds things pretty much as he expected to: Keith and Lance both have their bayards out and appear to be doing their level best to kill one another. As Shiro observes, Lance lays down a track of fire that Keith flips himself out of the way of. He lands and launches himself into the air again in the same motion, diving for Lance and coming up inside his guard. The hand-to-hand lessons Shiro's been trying to hammer into Lance's head are finally starting to pay off; Lance dances back, deflecting Keith's sword with his rifle, and uses the movement to drive in with his fist. It's a good move, but he's trying it on Keith, who grabs his wrist and uses Lance's own momentum against him to pitch him over his shoulder.

Lance lands with an audible _whumpf_ of the breath leaving his lungs, but he keeps his grip on his bayard. He really is improving, Shiro notes, pleased to see it.

Keith is on him before Lance can recover—reflexes are an ongoing project—and pins Lance, kneeling on his chest and shoving his blade under Lance's chin. Lance growls something that Shiro decides he didn't actually hear (there are only so many battles one man can fight) and Keith says, triumphant and breathless, "And _that's_ why I'm Shiro's favorite."

Wait, _what_?

While Shiro is trying to make sense of that, Lance sputters some and comes up with, "You _wish_!" which is honestly one of his better efforts.

Keith barks a short laugh at him. "No, _you_ wish _._ "

Lance growls another of those curse words that Shiro is steadfastly ignoring and manages to buck Keith off his chest. With that they're at it again, so focused on one another that they still haven't noticed that Shiro's standing there gaping at them. He knows—Lord, does he know—that Keith and Lance will fight over anything, the last serving of space goo, allegedly funny looks given or received, the fact that it's Tuesday, any excuse will do to set them off, but—they can't seriously be fighting over which one of them is his favorite, can they?

Shiro shakes his head to clear it; of course they can. They're in the middle of doing that very thing right before his eyes.

He makes a mental note to ask the princess to step up the planning of their next mission. It's clearly been too long since the last time they've gone out against the Galra if Lance and Keith are this desperate for an excuse to fight.

At this point, Lance has a stroke of luck and manages to crash into Keith while dodging his blade. It's neither graceful nor elegant, but it _is_ effective; they both go down, Lance on top, and he crows in delight. " _Now_ who's the best?"

"Still me," Keith says, much to Lance's indignation.

That's more than enough arguing for one day, and never mind what started it. Shiro is just about to step in and put an end to their playtime when Keith drops his bayard and reaches up to grab a fistful of Lance's t-shirt ("Hey!"). "Shut up already," Keith tells him as he hauls Lance down to him and seals their mouths together.

Lance goes with it, dropping his bayard and planting his hands against the deck on either side of Keith's head, returning the kiss as enthusiastically as he'd been trying to kill Keith just a minute ago.

Okay. Okay, so—that—is a thing that appears to be happening. And not for the first time, either, if the comfortable way Keith slides his hand down into the back pocket of Lance's jeans and the way Lance shifts and fits himself against Keith are any sign. It's a thing that is happening, and as one of them makes a throaty sound that the other answers, Shiro decides that retreat is very much the better part of valor. He backs out of the training deck as silently as he can, doing his best _not to see_ where their hands are going, and leans against the bulkhead once he's safely in the corridor.

That's what he tries to do, anyway. The problem with backing up is that he doesn't have eyes in the back of his head, so he doesn't see the gouge in the deck from some previous training session. When his heel catches it, Shiro lands on his ass hard enough to shake the room.

"Shit," he says as Lance pulls away from Keith's mouth and both of them see him sitting there. "Um. Hi, guys."

The three of them stare at one another for a minute, frozen by the awkwardness of the situation. Lance has a hand under Keith's shirt; Keith has both hands on Lance's ass. Shiro doesn't know where to put his eyes.

The only consolation in any of this, if there can be such a thing, is that Keith and Lance both are about as red-faced as Shiro feels.

Shiro clears his throat; the sound seems unnaturally loud, bouncing off the walls and making the two of them jump. "Sorry," he says. "I—sorry, I didn't mean to—I didn't know you were—" Were what? Together? Involved? "—busy," he finishes, clumsily, and scrambles to his feet. "I'll just—I'll be going, carry on. Or not. Maybe not on the training deck?" He shuts his mouth and sets his teeth on the inside of his cheek to shut himself up, because he's _babbling_. He's stumbled onto something that is none of his business and he really needs to shut up and leave and then go see whether there's some kind of Altean equivalent to hard liquor and if so, where he can get his hands on enough to remove this entire traumatic experience from his brain. "Sorry. Going now."

"Wait," Keith says before he can turn and go. "Shiro. Please." He takes his hands off Lance, who rolls off him so Keith can get on his feet unencumbered. Both of them tug at their shirts, though it doesn't do much for their modesty at this point. It's still painfully clear that they're both still—excited.

If there's not an Altean equivalent to hard liquor, by God, Shiro is going to cannibalize the castle-ship for parts and _build_ a still himself.

In the meantime, he's their commanding officer; Shiro pulls himself together and holds up his hand, keeping his eyes firmly above their shoulders. "I'm sorry," he says again. "I didn't mean to intrude on your privacy. Please forgive me."

He doesn't know what to make of the expression on Keith's face; there's embarrassment there, sure, but other things, too, things that Shiro's not sure how to name. It strikes him, all at once, how much Keith has grown up—not just since Voltron, but since the last time he saw Keith before Kerberos. And not just in the obvious ways, either. He's settled in himself now the way he wasn't then, sure and confident.

It's that confidence that's carrying him forward now, a kind of determination that can't be hampered by embarrassment. Keith crosses the deck to where Shiro is standing. "You don't understand," he says.

"No, I do understand." Shiro finds a smile for him, God only knows how. "I do. I'm surprised, don't get me wrong, but I understand. I'm happy for you." It's the truth; good things are difficult to come by in the best of times, which these aren't. Any kind of happiness a person can find in the middle of the war is a blessing, and Shiro isn't going to begrudge Keith that. "Congratulations."

Keith scowls at him—familiar frustration—and huffs. "No, you _don't_." He steps closer and slides his hand up Shiro's chest, over his shoulder and up, until he's cradling the back of Shiro's head. "It's not what you think."

And then Keith kisses him.

 _Keith is kissing him._

Shiro freezes—just freezes, and he's not going to be proud of that later, but he'd very much like to know what else he's supposed to do in a situation like this. They don't exactly cover this kind of thing in flight school.

Except that they actually do.

Shiro pushes aside the urge to lean into Keith's mouth—he can barely remember the last time he kissed someone, that was something that had been a part of his life before and has had nothing to do with his life after—and sets the hand that's still his on Keith's shoulder, pressing him back. "No," he says, as gently as he can manage. "We can't."

Keith gives him another of those frustrated looks. "Shiro—"

He makes as though to step closer, and Shiro has to tighten his grip on Keith's shoulder to keep him at a safe distance. "We _can't_ ," Shiro says again, feeling about as helpless as he ever has at the trace of hurt in the way Keith looks at him.

"Sure you can," Lance drawls, startling Shiro, who's all but forgotten his presence. He's still sitting, legs stretched out before him, leaning back on his hands. He's wearing a smile, smaller and more real, somehow, than his usual grin. "Did you forget? We're not in Kansas any more. I don't think the Garrison's rules about fraternization were written to account for magical lion robots or Voltron."

"They were written to keep officers from taking advantage of their subordinates," Shiro says, almost relieved. Being exasperated with Lance is something normal and familiar. Safe. "Magical lion robots don't matter, the fundamentals of the situation do."

Lance actually goes and rolls his eyes at him. "Okay, first of all, who are you calling subordinate? You're the black paladin, sure, but I seem to recall hearing somewhere that Voltron is a team effort and no one paladin's any better than any of the others." He grins as he tosses Shiro's own lecture back in his face, the little shit. " _Second_ , if anyone is the commanding officer around here, it's the princess, and we all know it. And third…" Lance's smug grin softens, turns a bit wry. "Dude. Shiro. Look at him. You can't take advantage of someone who knows exactly what he wants."

That's not _fair_. The entire thing isn't fair, especially not the way Lance has chosen to set aside his goofy airhead routine to show off the reason he was accepted as a Galaxy Garrison cadet in the first place.

Keith is still standing there, watching him and waiting, just as he always has. When Shiro meets his eyes, Keith tips his head to the side just a bit and reaches up to curl his fingers around Shiro's and loosen his grip. He keeps his eyes on Shiro's as he turns Shiro's hand in his and leans down to kiss his palm.

" _Keith_ ," Shiro says, protest and something else he doesn't want to name all tangled together. This isn't _fair_.

Keith rests his cheek against Shiro's palm; Shiro cups his fingers, almost against his will, fitting them to the line of Keith's jaw, the blade of his cheekbone, while Keith's hair slides across the back of his hand like silk.

Keith smiles at him, the barest upward tick of his mouth and the softening of his eyes. Shiro swallows hard, willing himself to be resolute, but this is an assault he's not built to withstand. He says Keith's name again, powerless to do otherwise, and finds himself sliding his hand into Keith's hair, drawing Keith back to him. Keith comes to him, easy as taking a breath, and fits himself to Shiro like he was made for it. "Shiro," he says, like a greeting and request and offer all at once, and wraps his arms around Shiro willingly as he tips his mouth up for a kiss.

And Shiro is still only—mostly—human.

He kisses Keith, leans into the softness and welcome of his mouth and the simple warmth of the act. He knows the barely audible sigh of satisfaction Keith lets out and can't keep himself from curling the other arm around Keith, gathering him in as he slips his tongue into Keith's mouth. Keith tightens his arms around Shiro and strokes his tongue against his with a pleased sound that verges on a moan. His eyes are shut, his lashes lying against his cheeks, and Shiro aches a little with how easily Keith trusts himself to this. To him.

Keith cracks his eyes open just enough that Shiro can see the softness of them behind his eyelashes, softness enough to make the breath catch in Shiro's throat.

He rests his forehead against Keith's, closes his eyes, tries to keep some kind of hold of himself and the ragged edges of his self-control. It would be so easy to let himself be swept away by the moment, to forget everything but the sweetness of it and how much he wants to surrender to it, but there are so many reasons he can't, the other paladins and the princess and—the other paladins. _Lance_.

Shiro is jerking his head back and looking before the thought even begins to register with him, so he's fast enough to see the look on Lance's face as he watches the two of them, apparent satisfaction tinged with wistfulness, the edges of the kind of regret Shiro can name all too easily—that's the look a person wears when he's giving something up that he knows wasn't ever really meant for him in the first place.

Their eyes meet while Keith is still making a startled sound at Shiro's sudden recoil; Lance widens his eyes a bit in the awkwardness at being caught watching something private. Then he crooks his mouth, small and rueful, and tips his head to him. Acknowledgment. Concession.

And that's not right, either.

The rules about fraternization aren't just about protecting one's subordinates from being exploited; they're about protecting everyone else under one's command, too, keeping from playing favorites and from excluding anyone.

He can't do this. He _can't_.

Keith turns in his arms just a bit, looking to see what Shiro is looking at (without loosening his grip on Shiro in the slightest). "Ah," he says, quiet, the tiny wrinkle of confusion between his eyebrows smoothing out. "Lance."

"I guess I'm the one intruding now," Lance says, breezy, climbing to his feet at the same time Shiro is saying, "We can't do this, the two of you already have each other."

Lance freezes for a split second, staring at Shiro, something like startled awe on his face. Then he shakes his head vigorously. "No, Shiro, c'mon, don't be like that, man." He spreads his hands, gesturing at Keith, at the way Keith is hanging onto him like grim death as Shiro tries to pry him loose. "The two of you, it's… it's _right_ , the way it's supposed to be, y'know? No way am I getting in the way of that, that's messing with destiny or something."

Shiro doesn't know about destiny— _really_ , Lance?—but he does know what's right. "No, you were here first, I can't—I won't—"

"No, I'm pretty sure you were here first," Lance says, sounding amused, maybe by the way Keith isn't letting go of Shiro despite the efforts he's putting into untangling himself.

Keith huffs. "Do _I_ get a say in this?" he demands. "Or are the two of you going to try and settle things without asking me what _I_ want?"

Shiro feels his face go hot, embarrassed, while Lance lets out a crack of laughter. "Guess that settles that."

"It doesn't settle anything." Keith scowls at Shiro and then Lance, dividing his irritation between them both.

Shiro can't ask for a better opening; he abandons the effort to free himself from Keith's determined grip and settles his hand on Keith's shoulder again. "Really," he says, quietly. "Keith, you and I can't do this, not when it means that you're choosing me over Lance."

Keith transfers his scowl back to Shiro. "Who says I'm choosing you over him? I want you both."

Shiro isn't so shocked by that blunt declaration that he can't hear the strong overtones of _you idiots_ attached to it.

While he's staring at Keith, Keith and the obstinate, flat line of his mouth, the expression he wears when he's set his sights on a goal and won't be turned aside from it, Lance laughs uneasily. "Dude. I don't think it works that way."

Keith turns to look at him. "Why the hell not?"

Lance opens his mouth, but seems to be at a loss for words (for once). He looks to Shiro, who would like to come to his aid, really, but is having just about as much trouble trying to figure out some way of explaining why the three of them _can't_ do what Keith is suggesting. Keith, who's always been just a little too ready to ignore any social convention he can't see the point of.

Keith snorts, craning his head to look between the two of them. "Yeah, that's what I thought."

"But it really doesn't work that way," Lance says, slowly. "I mean, it'd be nice, but c'mon, Keith, just how good do you think I'm going to be at sharing you with Shiro?" He rubs the back of his neck self-consciously. "You must think I'm way more mature than I actually am, and I have absolutely no idea what basis you have for that, all things considered. Though I'm flattered that you even think I can compete with him! That's probably the nicest thing you've ever said to me."

Shiro can see how hard Lance is trying to seem casual, but he wears the attitude like an ill-fitting suit. It's all too clear that he doesn't _feel_ casual about it at all, that he's trying to be generous and unselfish and _practical_ , regardless of what it's costing him.

God, how did he manage to be so lucky in this team?

"Lance," Shiro says before this can go too far, before Lance can't walk himself back.

Lance anticipates him, gives him something approximating his usual carefree grin. "Hey, just calling it like I see it."

Keith heaves an aggrieved sigh. "You're an _idiot_." He grabs the prosthetic arm and hooks his fingers between the metal ones, using it to haul Shiro across the deck to where Lance is standing. Shiro can't do anything but follow him, aggravated by the way Keith is using his own caution against him (even now he doesn't trust the prosthesis, doesn't trust himself with it) and admiring his sense of strategy. Keith grabs Lance, too, seizing his shoulder before he can retreat. "You're an idiot," he says again, but he sounds fond, not annoyed; in his mouth, _idiot_ sounds like an endearment.

Lance seems to recognize it as such, anyway, and smiles at him with a shrug. "Yeah, well…"

Keith sighs and hauls him in to kiss him, too.

Lance responds, perhaps not even thinking about it, lifting his hands and setting them on Keith's back, curling them in his shirt as he leans in and tips his head to fit his mouth against Keith's. He does it with unconscious, unthinking ease—how long have the two of them been together for them to be this comfortable in each other's space? The question aches, that this has been happening beneath his nose for so long without his noticing it—surely _that's_ why Shiro's throat feels tight, no other reason. What other reason does he have a right to claim?

Keith draws away from Lance's mouth with a soft hum, one of pleasure, leaving Lance leaning into him and showing things on his face better left in private. "Idiot," Keith tells him, fond. "Who said you were going to have to share _me_ , anyway? I thought we should share _Shiro_."

" _What_?" Shiro says, startled.

Keith hushes him—hushes _him_ —and tips his head at Lance, the challenge open.

Lance has never once in all the time that Shiro has known him managed to back down from a challenge, real or perceived; this time is no different. His eyes spark and he grins. "Well, if you wanna put it _that_ way…" He turns those hot eyes Shiro's way, all speculation. "Guess we could see who's really his favorite."

Shiro opens his mouth, though he doesn't have any idea which part of that he wants to tackle first, but Keith pokes Lance in the side first. "Be serious."

Lance grimaces at him. "You started it."

Keith gives him an annoyed look. "I did not!"

"Sure you did," Lance counters. "You said we should share him, and _that's_ not gonna happen. He'd have to want to be shared with me, which, yeah. I'm not seeing it."

Shiro is starting to understand why Keith was so frustrated by the way he and Lance had been talking about him; it's a very irritating experience. "I'm standing right here," he points out.

Lance gives him a wry grin. "Well, isn't it true? Keith's one thing, right, and I'm something else. Which, you know, I get it. No hard feelings."

Keith growls. "The two of you are too much alike." Shiro doesn't think they're doing much to undermine his point when they both sputter in unison. "You _are_. You're both so determined to give up without even trying! Like you think it'll be better that way. I don't get it!"

He wouldn't; Keith has always been straightforward that way, has never had to learn to be otherwise. Has never learned to compromise what he wants. Shiro knows that; when he finds himself sharing a look with Lance, he can see that he does, too.

But Keith isn't done with them, not yet. He shakes the prosthesis, Lance's shoulder, dragging their attention back to him. "What makes you think Shiro looks at you any differently than he does me?"

Lance snorts. "Maybe because I've got eyes?"

"Then use them!" Keith flares. He rounds on Shiro, who doesn't know what he should be saying to this. "Tell him!" The edge in Keith's voice jolts through Shiro; he knows that tone, though he's almost never known Keith to show desperation so clearly. "Be just Shiro for once and tell him!"

Just Shiro, he says, as though that were a simple thing—that easy to set aside all his responsibilities for the sake of—Shiro catches himself on that thought, catches himself on the way Lance is smiling, crooked and fatalistic, on the memory of the argument (or whatever it was) he'd overheard at the start of this entire awkward encounter. Keith knows him far too well, knows exactly how far he's willing to go for something that's his, whether he admits it out loud or even to himself. If it's for something—someone—who's his, Shiro can do anything.

And Lance is his, the same way Keith is, the same way the entire team is. They're his, his people, and set next to that, nothing else matters, not fraternization rules and not social conventions, not even his own unworthiness of such grace.

That's all there is to it. Lance is his, Lance is going wanting and believes that's the natural order of things, and that is _not acceptable_.

Shiro clears his throat and stretches his hand out to Lance as the way the world works spins and settles around him, the pieces clicking together to form new shapes. "Come here."

Lance gives him a blank stare. "Um?"

He doesn't move, so Shiro repeats himself, more firmly this time. "Come here, Lance." Lance's eyes go wide; he takes a step closer, close enough that Shiro is able to rest his hand on Lance's shoulder and draw him the rest of the way in, closing their little circuit. " _Is_ this something you really want?"

"I can't believe you think you even have to ask that." Lance's voice is unsteady as he strives for levity without success. "Geez, Shiro."

"I think I really do." Shiro squeezes his shoulder gently, conscious of the way Keith is watching them both. "Tell me what you want. Please."

"How long have you got?" Lance asks him. It should be a joke, but it comes out serious, uncertain.

"As long as you need," Shiro promises him. "I'm listening."

Lance's throat moves as he swallows. "You don't have to do this," he tries, not quite meeting Shiro's eyes any more. "I mean, you know. If you and him want to do your thing, and you don't mind him and me being a thing sometimes. I'm not sure how well that's gonna _work_ , mind you, but I'll give it a shot."

He's trying so hard, giving everything he's got to do what he thinks is right. If that isn't evidence that he deserves more than what he thinks he's going to get, Shiro doesn't know what is. He tightens his hand on Lance's shoulder. "Lance. You're not answering my question."

Lance twitches, obviously a guilty reflex. "Sounded like an answer to me," he mutters.

Shiro sighs and pulls him closer, then sets his fingers on Lance's chin, lifting it and holding him so he can't look away. "I know you know it wasn't. So let me ask you again: What is it that _you_ want?"

Lance makes a small, protesting sound, practically a whine. "Shiro…" Shiro waits, holding him and keeping his eyes locked on Lance's, watching him struggle with himself, and sees the moment when he surrenders by the way his expression shifts and goes bleak. "Everything," he says. "I want _everything_ , you and him and for this fucking war to be over and to go home to my family and a goddamn _pizza_ , I'm so sick of space food, none of it is _right_ , I want _everything_ and I'm not going to get it, I know that, but I was doing a pretty good job of not thinking about it till now, so I hope you're satisfied."

Keith grunts at the torrent of words, protesting or maybe just surprised, but stops when Shiro uses the prosthetic to squeeze his fingers, carefully. He holds Lance's defiant, ashamed gaze steadily. "I understand."

Lance scoffs, not that Shiro can blame him for it. "Sure you do."

"Listen to me," Shiro says before they can get off track, before Lance can work himself up into a righteous fit of anger (anger is so much easier than shame). He takes a deep breath. "I was doing a pretty good job not thinking about all the things I want, too. And then someone shoved my face in the fact that I could have some of them, if I wanted. If I was brave enough. So yeah. I understand."

Lance stares at him, the incipient burst of temper averted by his surprise. He starts to speak and then stops himself like he's not sure what to say. He swallows, looking at Shiro, eyes wide and lost. "Shiro…"

"I don't really know how this is going to work, either," Shiro confesses—to Lance, to Keith. To himself. "But I'm willing to try figuring it out if you are."

Lance opens his mouth and draws a shaky breath. "This is nuts. You're nuts." He shakes his head before either of them can react. " _I'm_ nuts. I know I'm nuts, because I want to try. Even though there's no way this is going to work."

Keith snorts. "There's no way magic, psychic robot lions should work, either, but that hasn't stopped us yet."

Lance closes his eyes and laughs, helpless, maybe a little hysterical. "Okay, so that's true," he admits. "Okay. Why the hell not. Let's give it a shot."

" _Finally_ ," Keith says, dour, but his tone is belied by the upward curl of his lips. "That was much, much harder than it had to be."

It really, really wasn't, but that's a conversation for another time.

Before Lance can open his mouth to argue with him—Shiro can see him ready to do it, reflexive as a heartbeat—Shiro kisses him. It startles a sound out of him, surprised, and for a moment he's still, maybe with shock. Then his eyes drift closed and he leans into Shiro, softening against him, opening for him and humming against his mouth.

It's easy, easier than Shiro would have thought it could be, to draw Lance to him, to wrap an arm around him at the same time Keith is slipping himself under the other. Lance comes willingly, eagerly, pressing himself against the two of them. The three of them fit so well together that Shiro almost can't believe this isn't something he's dreaming (but his dreams are never so improbable and rarely so pleasant).

Maybe, he thinks, as Lance draws back, smiling soft and dazed as Keith gives them both a satisfied look, maybe he'll take this as a sign that this thing isn't as crazy or impossible as it seems.

Yeah. He could live with that.

 **end**

As always, comments are a thing of utter delight.


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